This invention relates generally to gas flow measuring systems, and more particularly it relates to such systems of the open-end flow type, in which the flow in a conduit, known as a flow head, passes a restriction or obstruction in order to develop a pressure signal in proportion to flow through the conduit. Such systems have particular application in devices, known as spirometers, used for measuring pulmonary function.
In gas flow measuring systems of the type described generally above, and particularly in spirometers, the pressure differential developed in the flow head is communicated to a differential-pressure transducer which, in turn, produces an electrical signal which can be processed to yield values of flow rate and total flow volume through the flow head. In many applications, and particularly in spirometers, high degrees of accuracy are desired. It then becomes necessary to minimize errors which may be introduced into the system by the effects of ambient temperature changes and/or hysteresis on the mechanical components of the differential pressure transducer. Specifically, the typical pressure transducer provides for the pre-establishment of a point of reference from which to quantify the pressure measurement. However, this reference point can be caused to drift by such phenomena as hysteresis and ambient temperature changes, with resulting error in the pressure measurement. If, however, the reference point is sampled and re-established at intervals well within the time frame of the reference-point drift, suitable accuracy can be maintained.
The prior art has taken a number of approaches toward solving this problem of "zero drift." One approach has been the use of a mechanism which periodically "grounds" a pressure transducer to a predetermined reference pressure, while simultaneously driving the transducer output signal to a selected reference level. Examples of devices of this type are disclosed in U.S. Pat. No. 3,509,767--Greer; U.S. Pat. No. 3,765,239--Olsson: U.S. Pat. No. 3,924,612--Dempster et al.; and United Kingdom Pat. No. 850,750.
Another approach has been the use of a differential pressure transducer along with valving means for periodically reversing the pressure inputs to the transducer so as to produce an A.C. signal at the transducer's output. Electronic means are then used to determine the peak-to-peak value of the A.C. signal, the effects of zero drift thereby being nullified. Examples of this approach are found in U.S. Pat. No. 3,296,868--Koppel et al.; and U.S. Pat. No. 4,271,701--Dempster et al.
A common aspect of both of the approaches outlined above is the use of some valving mechanism which periodically and at regular intervals either "grounds" the transducer or reverses the "polarity" of the pressure inputs. In both cases, some means is required for detecting the state of this valving mechanism so that the reference point of the transducer output signal may be sampled and re-established in proper synchronism. Typically, such valve state detection mechanisms utilize optical, magnetic, and/or mechanical means for sensing the valve state, and some means for generating a valve state indicative signal for input to the synchronization circuit.
It will be appreciated that the need for such valve state detection mechanisms adds to the complexity of the overall apparatus, thereby increasing the costs thereof. Moreover, the accuracy and reliability of the valve state detection mechanism affects the overall accuracy and reliability of the gas flow measuring system in which it is incorporated. In addition, in such prior art devices, two separate mechanisms are used for, respectively, actuating the valving mechanism and detecting the valve state. This introduces another synchronization problem, with possible adverse effects on the reliability and accuracy of the overall system.
It will thus be appreciated that the need has been established for a gas flow measuring system capable of enhanced accuarcy and reliability through the simplification of the valve state detection mechanism and through the enhancement of the inherent accuracy of this mechanism. It will also be appreciated that it would be desirable to provide such enhanced reliability and accuracy with a mechanism which is relatively inexpensive to manufacture and which is simple to maintain in proper working condition.